Is there a limit to the number of hours a researcher can work on a Marie Curie project especially if this researcher is involved in parallel in more than one EC Funded projects?

In principal, the researcher recruited for the MSCA project should devote him/herself for this project on a full-time basis, which means that s/he cannot be employed for another project. Researcher could be employed for part-time only in special circumstances, usually due to personal/family reasons and this always needs to be approved by REA PO.
The number of hours equal to full-time employment is determined by national law, sometimes internal regulations of the institution.